1. Field of the Invention
The present invention relates to a radio apparatus.
2. Description of the Related Art
Japanese patent No. 3419484 and JP 2005-341240 A discloses background art in the field of the invention.
Japanese patent No. 3419484 discloses a modulator including a signal converting means responsive to a modulation signal for conversion to have a digital selection signal which becomes an predetermined analog signal when time averaging is made and a selecting means for selecting one of signals having different phases or amplitudes with the same frequency on the basis of the selection signal to output the selected one.
JP 2005-341240 A discloses a technology of radio apparatus in the abstract as follows:
A lower harmonic wave removal device 20 supplied with a baseband signal and a reference clock signal having a period shorter than the period of the baseband signal outputs a voltage which is derived by approximating a sine wave synchronizing with the reference clock signal in accordance with variation in codes in the baseband signal. The analog filter 22 removes harmonic components from the output signal of the lower harmonic wave removal device.
However, in Japanese patent No. 3419484 and JP 2005-341240 A, there are problems as follows:
Japanese patent No. 3419484 disclosed a method of limiting a band of the baseband signal with a ΔΣ modulator.
In this method, it is impossible to stop frequency components near the carrier frequency. Accordingly there is a problem in that a circuit scale becomes large because it is necessary to add filters for restricting adjacent channel leakage power.
JP 2005-341240 A discloses a method of removing high frequency components of the baseband signal using analog switches. However, this method requires amplitude modulation and analog switches are necessary outside the digital circuit. Accordingly, there is a problem in that the circuit scale becomes large.